The present invention relates generally to a printing plate exposure apparatus for recording image data of printing image signals from a computer on printing plates on curved surfaces of drums.
Prior Art
Recently, with advances in the technologies of data processing and image processing by computers, a method of effecting a drawing directly on a printing plate with a laser beam by use of image signals from a computer, without interposition of photographic film, has been invented and put to practical use, even in the field of newspaper printing and the like.
Also, a well known printing plate called a PS plate--i.e., a printing plate for which a photosensitive material is pre-coated on an aluminum base plate--is commonly used.
Conventionally a well known drum rotary system for facsimile apparatus is used as a mechanism for drawing on such printing plates. Although a laser beam is used for drawing on the printing plate on the drum, technology has advanced such that a system of using a plurality of beams, rather than one beam, has been adopted for increasing drawing speed. Hence, as a conventional example, a system of recording data in a band shape with a plurality of laser beams, is described hereinafter.
As described above, a multi-beam system shown in FIG. 11 is known as an apparatus for drawing picture images on a printing plate. Reference numeral 2 is a horizontally arranged drum 2 to be rotationally driven in a direction of an arrow "a" by a motor M, with a printing plate 4 being wound on an external circumferential face of the drum 2.
Since the printing plate 4 is wound in a regular cylindrical shape such that a start side edge line 6 of the printing plate 4 and an end side edge line 8 thereof are not skewed, a top edge line 10 of the printing plate and a bottom edge line 12 thereof are positioned in parallel to each other in a drum axial direction.
An optical apparatus is mounted on an optical stand 14. A laser beam coming from a laser light source 16, such as a semiconductor laser or the like, is divided into multi-beams by a multi-beam producing element 18, such as a grating, a Wollaston polarizing prism or the like. The multi-beam passes through an AOM group (acoustical optical modulator) 20 which is switch-controlled by image signals from the computer C. Then, the beams enter into an exposure unit 22 including a light source array made of optical fibers and are irradiated onto the printing plate 4 through a lens 24. The light source array 22 is an example of an exposure unit for directly recording images onto the printing plate. An image band 26 is recorded on the printing plate 4 within a band width B corresponding to an irradiation width of the multi-beam. A moving mechanism H causes the optical stand 14 to continuously move at a uniform speed in a direction of an arrow "b" a distance of the band width B for one revolution of the drum 2.
Problems to be Solved by the Invention
The aforementioned exposure apparatus has the following problems. As shown in FIG. 12, the drum 2 rotates at a uniform speed while the optical stand 14 moves the distance of the band width B per revolution. Therefore, the image band 26 is spirally recorded on the printing plate 4, axially shifting a distance of the band width B for one revolution. When the printing plate 4 is unfolded (flattened), a recording image 28, composed of many image bands 26 is formed in the shape of a parallelogram, distorted by a distance of the band width B due to the spiral recording operation. The printing plate, having such a deformation of the recording image 28 relative to the printing plate 4, cannot be adopted for printing use.
For a printing plate for newspaper use, when a circumference of the drum is 46 inches, a picture element density is 909 dots per inch and a light source array 22 has 128 beams, the band width B is the equivalent of 128 picture elements, i.e. 128/909=0.14 inch, or 3.6 mm. The spiral recording results in a distortion of 3.6 mm, which is not visually negligible, between a top edge line 28c of the recording image 28 and a bottom edge line 28d thereof. An inclination angle .theta. is 0.17 degrees, as calculated from tan .theta.=0.14/46, and hence, a start side edge line 28a and an end side edge line 28b, both parallel to each other, have the inclination of 0.17 degrees.
In order to remove the inclination of the start side edge line 28a of the image with respect to the start side edge line 6 of the printing plate 4, an intermittent moving system shown in FIG. 13 has been invented. Namely, movement of the optical stand 14 is stopped while one revolution of the drum 2 for image recording takes place. Then, the optical stand 14 is moved through a distance of the band width B by the moving mechanism H. The moving time has to be set at an integral multiple of time required for one revolution of the drum. If this intermittent moving system is adopted, a recording image 28 is recorded in a regular square shape 2. The recording image 28 is not distorted with respect to the printing plate 2, because the image bands 26 are formed in regular circumferential shapes, not in the spiral shapes. But, the intermittent movement has a problem in that efficiency of such a plate making operation is lowered because an overall exposure time is several times longer than for the spiral recording method.
Also, a necessity for repeatedly starting and stopping movement of the heavy optical system makes the apparatus complicated and higher in price. This also causes vibrations which are likely to cause deterioration of images and malfunctions of the apparatus.